Ultrasound shear wave elastography is a new medical modality that can measure mechanical properties of soft tissue such as shear modulus and shear viscosity. As tissue elasticity is related to pathology, elastography can provide additional clinical information to increase diagnostic confidence. One example is non-invasive liver fibrosis staging by measuring liver stiffness.
In acoustic-radiation-force based ultrasound shear wave elastography, the dedicated pulse sequence consists of one or more long push pulses (typically hundreds of microseconds long each) and a series of interleaved tracking pulses. Due to the effect of acoustic radiation force, the push pulse causes tissue in the focal area to move away from the probe surface, simultaneously establishing a shear wave propagating away from the focal region in a direction perpendicular to the push beam. For each lateral position along the shear wave pathway at the focal depth, the tissue motion induced by the shear wave will be mainly in the same direction as the push beam. Consequently, the tracking pulses can, for a given position, monitor such dynamic response and translate it into a position-specific displacement waveform representing the magnitude of tissue movement as a function of time. Such waveforms can be computed at multiple positions along the shear wave propagation path. The waveforms can be inputted into a process for calculating the speed of the propagation. For example, Fourier transform can be performed on those shear waveforms to estimate shear wave phase velocity. Alternatively, shear wave amplitude peak-to-peak spatial and temporal calculations can also yield the shear wave propagation speed. As a result, absolute values of tissue mechanical properties can be determined. In particular and by way of example, the speed at which a shear wave propagates inside the tissue is governed by shear modulus, shear viscosity, tissue density and shear wave frequency through some mechanical models. The stiffer the tissue is, the faster the waves move. A measure of the stiffness can then be used in, for example, liver fibrosis staging.
Accurate, reliable and efficient motion tracking is a goal in the final estimation of tissue properties in any form of ultrasound shear wave elastography. In general, there are two major motion tracking techniques in ultrasound imaging: 1) phase shift by auto-correlation, and 2) time-shift by cross-correlation (or other alternatives such as sum absolute difference (SAD)).
In autocorrelation based approaches, the displacement of the structures of interest induces phase shift on successive high frequency ultrasound echoes backscattered by the moving medium. Autocorrelation has been implemented on most commercial ultrasound systems for real-time color flow imaging. As ultrasound waves propagate through soft tissue, the spectrum experiences a downshift due to the frequency dependent attenuation. Therefore, assuming a constant center frequency in Doppler-based approaches will result in displacement estimation error. It is known to use additional autocorrelation in the fast time domain, i.e., in the axial direction of ultrasound pulses, to estimate the local center frequency and subsequently improve the displacement estimation accuracy. See T. Loupas, J. T. Powers, and R. W. Gill, “An Axial Velocity Estimator for Ultrasound Blood Flow Imaging, Based on a Full Evaluation of the Doppler Equation by Means of Two-Dimensional Autocorrelation Approach,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 42.4., pp. 672-688, 1995.
Time-shift by cross-correlation estimates time delays by cross-correlating, from one pulse to another pulse using radiofrequency (RF) data or complex signals conveyed from RF data. The search area is preset large enough to avoid missing an optimal match.